A typical thickener takes the form of an upright cylindrical tank having a feed well situated concentrically within it. Feedstock in the form of a slurry is introduced into the feed well by way of a feed channel. A controlled amount of supernatant liquid is also introduced into the feed well to dilute the feedstock if required. During operation of the thickener tank, a flocculent is introduced into the diluted feedstock to enhance settling of the mineral ore in the tank, so that solids settle to a compaction zone towards the bottom of the tank and flocculated material settles in a hindered settling zone which forms above the compaction zone. Supernatant liquid accumulates in the tank above the hindered settling zone and is removed by way of an overflow launder which encircles an upper portion of the tank.
The floor of the tank is of frusto-conical shape and slopes downwardly towards a centrally-located underflow outlet. Solids collecting on the floor of the tank are raked gently towards the outlet and are removed through it.
The use of flocculants will, in many cases, greatly enhance the settling rates of the solids through the tank. This allows the tank size for a given installation to be reduced. The process of flocculation may also often be improved by dilution of the feedstock. However as the feedstock density is generally determined by processes occurring upstream of the thickening tank, several techniques have been developed to dilute the feedstock by transferring back into it some of the supernatant liquid collected in the upper portion of the tank or in the overflow launder.
One such technique involves the use of the difference in densities of the liquid in the thickener tank, and the feedstock in the feed well. The transfer of liquid from the tank to the feed well then takes place by gravity
Another technique involves pumping supernatant liquid collected in the peripheral overflow launder encircling the upper portion of the thickener tank, back into the stream of feed slurry flowing towards the tank.
Yet a third technique is to pump supernatant liquid from the thickener tank back into the stream of feed slurry flowing towards the tank.
Each of the above techniques proposed or used in the prior art has attendant disadvantages. The use of gravity and differential densities, although simple, lacks good control. This technique also is dependant upon the prevailing operating conditions of the installation and often mitigates against the transfer of large volumes of supernatant liquid from the thickener tank to the stream of feed slurry flowing towards it. The technique involving the pumping of dilution liquid collected in the peripheral overflow launder surrounding the tank, involves additional expense. This results from the increased constructional costs associated with the provision of a larger tank together with additional pipe work extending between the launder and the feed well. Finally the technique of submerging or partly-submerging pumping apparatus inside the thickening tank in one or more positions can disrupt the smooth flow of liquid through the tank and traveling radially outwards towards the overflow launder. This follows from the risk of occurrence of changing, highly localized, rising flow rates occurring inside regions of the tank. These can adversely affect the clarity of the supernatant liquid entering the overflow launder with consequential operational problems caused by the changing liquid flow patterns inside the tank with different levels of operation of the pumping apparatus. For instance, the location of the pumping apparatus above the hindered settling zone will result in the smooth, radially-outward flow of supernatant liquid towards the launder being disrupted by regions of upward suction of the supernatant liquid, which occur beneath the locations of the pumping apparatus. Such disruption can result in a thick and even layer of flocculated material residing in the hindered settling zone being partially broken up and, as a consequence, some of the flocculated material being carried with the supernatant liquid into the overflow launder to adversely affect its clarity as well as the efficient working of the thickening tank through regions of depletion of the flocculated layer in the hindered settling zone.